In this specification, the term "hollow fiber" refers to fibers of a generally tubular shape having a continuous passageway (or lumen) disposed substantially along the axial center line of the fiber. The term "membrane" refers to a porous or microporous material, typically a polymeric material, which may be in the shape of a hollow fiber or a film.
The term "spiral" or "spiral wound" refers to membranes or membrane separation devices wherein the membranes are in the form of an asymmetric film or thin film composite material which is wrapped compactly around a central support.
Hollow fiber bundles comprise a plurality of hollow, porous fibers which can be arranged in the shell or vessel. The hollow fiber membranes provide a large overall surface area for contact with the feed water.
A fluid separation apparatus comprises a pressure vessel which houses one or more bundles of membranes. The fluid separation apparatus separates fluids by using a membrane having a selective permeability and may be applied to various techniques such as gas permeation, liquid permeation, dialysis, softening, ultrafiltration, reverse osmosis, or the like. Recently, attention has been particularly given to the reverse osmosis which is especially effective for desalination and purification of sea water or brackish water, for recovering useful or harmful components from waste water or for reuse of water.
Fluid separation apparatuses are generally classified into flat membrane type, tubular type, spiral type and hollow fiber type according to the shape and form of the semi-permeable membrane used therein. Among these, the hollow fiber type and the spiral type membranes are especially well known in the art, particularly for reverse osmosis separations, such as desalination of sea water or brackish water and purification of wastewater. In fluid separation apparatuses which incorporate spiral wound membranes, the membranes have a disadvantageously large spacing between the membranes, often ten to twenty times greater than the spacing for comparable hollow fiber membranes. Moreover, the pressure vessels used for spiral wound membranes tend to be very long in order to operate with desirably high feed flow rates, at high conversion and prevent disadvantageous concentration polarization. Pressure vessels for spiral wound membranes are known to be more than twenty feet long. The flow path for the fluid flowing on the reject side of the membrane and through the pressure vessel is, therefore, also very long, which requires the large spacing between membranes, so as to enable reasonable pressure drops across the length of the pressure vessel.
Hollow fiber fluid separation apparatuses solve most of these problems associated with spiral wound fluid separation apparatuses. For example, the flow paths for the fluid on the reject side of the membrane are relatively short, the radial flow path is short and the pressure drop across the bundle and down the annulus along the length of the device is not excessive. In addition, the spacing between hollow fiber membranes is typically small (generally about 25 microns vs. 25 mils for spiral wound membranes). As a result, a hollow fiber type apparatus has very high membrane separation efficiency per unit volume of the apparatus. Hollow fiber membranes are particularly suited for reverse osmosis separations.
The use of the reverse osmosis membrane inherently requires appropriate membrane housing and associated plumbing connections to handle three separate water flows. Specifically, the membrane housing and plumbing connections must accommodate connection of the feed fluid to the membrane, as well as flow of the purified (permeate) and reject (residue) fluids from the membrane. In the past, spiral reverse osmosis membranes have been provided in a cartridge form with a view toward facilitated cartridge replacement on a periodic basis, but prior reverse osmosis systems have not provided any optimally simplified cartridges, especially hollow fiber cartridges, for quickly and easily installing and removing the cartridge in a substantially fail-safe, error-free manner.
The reverse osmosis is usually carried out by treating a fluid under pressure higher than the osmotic pressure of the fluid, by which the components of fluid are separated via a membrane having a selective permeability. The feed pressure may vary with the kinds of fluids to be treated, the properties of the selectively permeable membranes, or the like, but is usually in the range from 10 to 1000 psig for spiral wound membranes and from 40 to 2,000 psig for hollow fiber membranes.
The prior art describes hollow fiber membrane type fluid separation apparatuses where at least one pair of hollow fiber bundles are contained within a vessel; however, fluid separation apparatuses housing multiple hollow fiber bundles, especially more than two bundles, are not well known in practice. These devices may require complicated hardware and the fluid to be treated is separated by bundles typically in series, thereby disadvantageously reducing the volumetric efficiency of the apparatus. The hardware typically includes at least two O-rings at each end of the bundle of hollow fiber membranes. The O-rings form a seal between the outside of the bundle and the inner surface of the pressure vessel. The O-rings serve to hold the bundle in place within the pressure vessel and to seal various fluid streams from each other during operation of the device. The O-ring seals and the complicated hardware makes it difficult to install or replace the hollow fiber bundles in the pressure vessel.
It is also known that failure of the hollow fiber membranes may necessitate prompt replacement of a bundle. For example, the fibers are fragile and may be easily damaged during transport, handling, assembly and operation of the fluid separation apparatus; as soon as fibers break or develop a fault, it may be necessary to replace the bundle of hollow fibers. Moreover, repair of damaged fiber is not economic. Therefore, it is highly advantageous that the hollow fiber bundles be readily installed and replaceable within the pressure vessel. However, as noted above, typically hollow fiber bundles use O-rings at the vessel inner diameter to seal the low pressure permeate from the high pressure feed fluid. This is achieved with a series of O-rings at each end of the bundle. The O-ring/bundle assembly must then be inserted with considerable mechanical force into the pressure vessel. Fiber bundle inspection and replacement is, as a consequence, difficult.
Existing designs have many other disadvantages. For example, in many applications (such as shipboard or portable use) the maximum possible area of membrane must be contained in the smallest possible volume and it is desirable to have greater flow per unit volume of the device. Pressure vessels which house only one bundle of hollow fiber membranes require excessive external piping which is costly to install and takes up space.
There is also a need for compact transportable equipment for mobile or military use. There is also a need that such equipment be at least partially assembled during transport and that it be easy to complete the assembly for rapid use in the field.
Furthermore, it is desirable to have a range of sizes and dimensions of hollow fiber bundles made available for different applications. Varying feedstocks to be treated contain different amounts of impurities and for economy, those with few impurities should be treated at high flux rates through the membrane. Long cartridges containing fine hollow fibers are not able to provide a high velocity of drawoff of permeate because of the hydraulic pressure drop of flow in the narrow lumens of the fibers and hence short cartridges are required. Conversely, some feeds require longer cartridges where the lower membrane flux rates present no problems of lumen pressure drop.
Another problem with prior art designs arises because different types and batches of fibers have different quality in terms of initial defects or service failure rates per unit of fiber surface area. There is also a risk of construction defects. It is, therefore, desirable to periodically test or inspect the membranes, which necessitates installing and removing numerous bundles from pressure vessels.
The testing of bundles for defects also presents problems. Bundles, especially spiral wound bundles, are typically tested for failure by means of a bubble pressure test. Existing hollow fiber bundles must be installed into a pressure vessel for such testing. During testing, when water occupies all of the pores in the membrane, a certain pressure, known as the bubble point of the membrane, has to be exceeded to overcome the interfacial tension of the water in the pores. In the bubble pressure test, air is forced back into the lumens of the wetted fiber. Failed fibers allow air to pass through the fiber walls at a pressure lower than the bubble point of the membrane. The opacity of prior art pressure vessels does not allow visual detection of a failed hollow fiber. This problem further magnifies the need for a hollow fiber bundle cartridge which may be readily installed into and removed from the pressure vessel and a hollow fiber cartridge which may be tested without installing it into a pressure vessel.
It is also desirable to have a hollow fiber membrane cartridge which may be readily installed in a pressure vessel which previously housed hollow fiber bundles or spiral wound elements. Unfortunately, in typical spiral wound devices the permeate fluid is discharged from a center tube. In typical hollow fiber devices the feed fluid is introduced through a center tube and the permeate fluid is discharged through another opening in the pressure vessel. Therefore, to retrofit an existing spiral wound bundle with a prior art hollow fiber bundle, it is necessary to use complicated pipes or fittings to "reverse" the flows of the feed fluid and the permeate fluid from the respective ports.
The present invention provides a hollow fiber membrane cartridge which keeps the advantages of the prior art and multiple cartridges may readily be installed in and removed from a pressure vessel. The cartridge is particularly suitable for use in pressure vessels that previously housed spiral wound elements. The inventive cartridge is a simple, economical device. The flow paths, are designed so that the permeate discharges through a center tube, which facilitates the installation of the inventive cartridges in a pressure vessel that previously housed spiral wound elements. Each cartridge is equipped with a tubesheet having individual pressure end caps which may be connected to appropriate ports or fittings to deliver the permeate fluid or the residue fluid and/or for making external fluid connections to other cartridges. These cartridges can be manufactured so that economy, convenience and utility can be optimized by varying the number of fibers per cartridge. These objectives, as well as other objects, features and advantages of the present invention, will become apparent to those skilled in the art from the following description.